This invention relates to a cooking vessel, particularly one having a bottom of an electrically conductive, non-magnetic material which can be coupled to the resonant circuit coil of an induction cooking or heating appliance.
Cooking or heating appliances of this type generate eddy currents electromagnetically in the bottom of the cooking vessel. The electrically conductive, non-magnetic material at the vessel bottom is usually composed of copper and serves as a heat conducting layer which has the purpose of uniformly distributing the heat, which is generated only in limited areas, over the bottom of the vessel.
The cooking vessel coupled to the resonant circuit coil corresponds to a load resistance which dampens the resonant circuit of the cooking appliance. In conventional cooking vessels, this load resistance is essentially ohmic. It has been found that the non-load power consumption of the resonant circuit network can be reduced and the loaded output which can be obtained from the resonant circuit network can be increased when the load of the cooking vessel has a complex impedance composed of resistive and reactive components by means of which the resonant circuit network is not only dampened, (i.e., the Q reduced,) but the resonant frequency or frequencies also shifted or detuned by the cooking vessel.
This principle is used in the appliance described in the aforementioned copending application of Rudi Tellert. This induction cooking appliance includes a sine wave generator in which a parallel resonant circuit is connected, in series with a series resonant circuit, to a square wave voltage generator. The cooking vessel can be coupled to the parallel resonant circuit. The resonant frequency of the parallel resonant circuit is essentially equal to the fundamental frequency of the square wave voltage generator and lower than the resonant frequency of the series resonant circuit. In this case, to be able to keep the non-load losses as low as possible, and to obtain a relatively high efficiency together with correspondingly low switching losses while loaded, the resonant frequency of the series resonant circuit is selected to be lower than the frequency of the third harmonic of the fundamental frequency of the square wave voltage generator. The resonant circuits and/or the complex load impedance composed of resistive and reactive components of the cooking vessel are dimensioned in such a way that, while the system is loaded, a phase displacement of about 20.degree. to 40.degree., preferably 30.degree., is obtained between the square wave voltage and the fundamental frequency component of the output current of the square wave voltage generator. The sum of the fundamental frequency component which is out of phase when the circuit is loaded, and the third harmonic which is continually, inductively out of phase by 90.degree. is no-load as well as load conditions, results in an output current from the square wave voltage generator which is almost rectangular and which lags the square wave voltage by 30.degree.. Accordingly, the square wave voltage generator is terminated almost ohmically (i.e., resistively,) while loaded, and delivers a maximum output to the resonant circuit network. During no-load, it produces a minimum output because the parallel resonant circuit adjusted to the fundamental frequency of the square wave voltage generator is blocked.
An object of the present invention is to improve inductively heated cooking vessels.
Another object of the invention is to provide a cooking vessel for an induction heating or cooking appliance, particularly an appliance of the type described above, in the aforementioned patent application, in which the resonant circuit network of this cooking appliance is shifted or detuned relative to the no-load condition so that optimum conditions prevail for extracting the output.